Settings should be stored through IntelliJ's API

[aurelienmazurie-wk]

The plugin is currently storing the URL for the sourcegraph instance in a file whose location is hardcoded to be the user's home directory (see https://github.com/sourcegraph/sourcegraph-jetbrains/blob/master/src/Util.java#L51). Not only does this unnecessarily pollutes the home directory it also ignores the fact that IntelliJ has a mechanism to persist plugin configuration variables.